The purpose of the project proposed here is to continue our research on the seasonality in births in human populations and to extend our analysis in several new directions. The results of our current project reinforce our view that analysis of birth seasonality can improve our understanding of both the behavioral and biological determinants of fertility. We have confirmed that strong seasonal patterns in fertility exist in virtually all human populations, both contemporary and historical. Our comparisons of seasonal patterns across populations appear inconsistent with a number of existing hypotheses about birth seasonality, while suggesting other hypotheses that require more direct testing. Our use of direct measures of the weather shows that inclusion of these variables explains some but by no means all of the seasonality of births. The purpose of the second phase of our project is to use additional sources of data to shed light on those hypotheses that cannot be ruled out by our existing results and to investigate the implications of these results for the general determinants of fertility. We will use monthly data on marriages, economic variables, and fetal and infant mortality to directly test a number of hypotheses about birth seasonality. We will also use survey data to examine directly the influence of economic and social variables on seasonality.